When normal subjects are given secretin before intravenous glucose (5g) pulses, insulin responses (within 5 minutes) are increased two-fold compared with saline controls (Journ. Clin. Endocr. Metab. 44:1-9, 1977). Diabetic subjects with decreased insulin responses also have a 2.5-fold improved response to 5g glucose pulses after secretin pretreatment. The acute insulin responses immediately following intravenous secretin are the same in both normal and diabetic subjects. Thus the ability to augment insulin response to glucose after secretin as well as the ability to release insulin directly after secretin is intact in diabetic subjects. Measurements of endogenous plasma secretin levels have been estimated by radioimmunoassay in normal and diabetic patients. Both groups have comparable values (8 pg/ml). Diabetics in ketoacidosis have the same levels as non-diabetic subjects. Thus endogenous secretin levels are indistinguishable from those observed in normal subjects.